Aircraft turbine engines typically employ starting systems which are separate from the electrical power generation system. Technology now exists which allows for the combination of these two functions into a single system comprising a generator which can be driven as a motor and an electronic power converter to perform the driving function. Auxiliary power units (APUs) are a subset of aircraft turbine engines which are also candidates for this combined starter/generator technology. In APU applications the turbine engine normally runs at a constant speed, driving the generator at a constant speed and, as such, does not require additional power conversion for the generate mode of operation. The start mode, however, requires a dedicated electronic converter to drive the generator as a motor to perform the start. The addition of this converter to the system is effectively offset in terms of cost, weight, and reliability by the elimination of the old dedicated starter motor and its associated gear train and mechanical disengagement device (clutch). Thus, there is a finite limit on the size and cost, as well as reliability, which the new converter must possess if it is to be a competitive candidate. A further requirement of this new APU starting system is that it must be capable of operating from the existing aircraft battery so that it will be compatible with existing systems and operational requirements.
Present APU starter generator system concepts are rather complex in order to satisfy all these requirements. For example, as illustrated in FIG. 1, a typical starter/generator system 100 requires three power converters (inverters), one for the motor torque generation/control function (main stator currents) 102, one for the exciter power generator control (main rotor current) 104, and an inverter 106 to boost the typical 28 Vdc battery voltage to a level compatible with the internal back EMF voltage produced by the starter/generator 108 during the starting process. Additionally, in APU applications, there is always a requirement to provide galvanic isolation between the input terminals of the starting system and its output terminals. This is because, in aircraft installations, the generator neutral and the battery negative terminal are permanently connected to ground (airframe) and isolation is necessary to prevent unwanted, non-torque producing, currents from flowing, thus providing maximum efficiency with minimal battery drain. Overall starter efficiency is of utmost importance in battery powered starting systems because of the limited amount of energy available from the battery.
It is not unusual for start systems with this type of architecture to be fiscally non-competitive with the existing DC motor starter systems, yet no one has come up with a suitable way to effectively improve upon this situation. The goal in the APU starter/generator concept is, therefore, to provide the best cost and efficiency by reducing the number of power converters and isolation transformers required. Achieving this goal will generally improve the size, weight, and efficiency of the system.